1. Field of the Invention
The present invention relates to a toner for developing a latent electrostatic image, a process cartridge and an image forming method.
2. Description of the Related Art
A developer used in electrophotography, electrostatic recording or electrostatic printing is, for example, once adhered to an image bearing member such as a photoconductor on which an electrostatic image is formed in a developing step, and transferred from the photoconductor to a transfer medium such as a transfer paper in a transferring step, and then fixed on the paper in a fixing step. As a developer to develop an electrostatic image formed on a surface of the latent electrostatic image bearing member, a two component developer containing a carrier and toner, and a one component developer containing a toner without carrier (a magnetic toner or non magnetic toner) have been known.
Conventionally, as a dry toner used in electrophotography, electrostatic recording or electrostatic printing, a toner, in which a toner binder such as a styrene resin or polyester is melted and kneaded with a colorant and pulverized, is used.
To obtain a high-grade and high-quality image in electrophotography, electrostatic recording or electrostatic printing, toners have been improved by reducing their particle diameters and narrowing their particle size distributions. However, a toner may have a indeterminate particle shape when produced using common kneading and pulverizing method, and in a machine, the toner is further pulverized by stirring with a carrier in a developing part, and by contacting with a developing roller, a toner supplying roller, a layer thickness regulation blade and a frictional electrification blade when the toner is used as a one component developer, and then submicron particles may be generated and a fluidizing agent is embedded in a toner surface, thereby reducing image quality. The toner has a poor fluidity as a powder because of its shape, and needs a large amount of a fluidizing agent, and has a poor filling rate in a toner bottle. This blocks downsizing of an image forming apparatus.
A transferring process of a formed image, which is composed of a multi-color toner to produce a full color image, from a photoconductor to a transfer medium or a paper has been becoming complicated. An indeterminate particle shape, such as that of a pulverized toner adversely affects transferring property, a transferred image may have a missing part and the large amount of toner is consumed to make up for the missing part.
Therefore, there has been an increasing demand for obtaining a high-grade image without a missing part with less amount of toner consumption by improvement of transferring efficiency, and for reduction of a running cost. The toner having an excellent transferring property does not need a cleaning unit to remove untransferred toner particles from a photoconductor and transfer medium, and achieves advantages, for example, an a downsized apparatus, low cost and no waste toner. In order to make up for disadvantages of the indeterminate particle shape, various methods for producing spherical toners have been invented.
For example, in addition to a suspension polymerization method, an emulsion polymerization method and dissolving suspension method in which particles are relatively-easily deformed have been proposed (See Japanese Patent Application Laid-Open (JP-A) Nos. 2-111967 and 2003-202708).
A mixture containing a toner raw material such as a binder resin or a monomer from which the binder resin is made, a colorant, a wax component and a charge control agent is dispersed in an aqueous medium to granulate particles of the mixture of the toner raw material, thereby obtaining toner particles having suitable size, specifically, having a diameter of approximately 3 μm to 12 μm. The obtained toner particles are taken out of the aqueous dispersion medium, washed, and dried to yield a toner.
However, in suspension polymerization, radical polymerization is generally performed and useful monomers are limited to carboxylic acids and esters thereof having a radical polymerizable unsaturated double bond, such as styrene, acrylic acid and methacrylic acid, in terms of cost. The styrene-acrylic resin to be polymerized can be designed to have suitable electrostatic property and fixing property, but not strong enough as a resin, and a toner may be broken and cracked by contact stress in a developing process. Toner fine powders generated by toner crack remain on a developing roller because they have large electrostatic or non-electrostatic adhesive force. The developing roller contaminated by the toner fine powders has less toner charging ability. As a result, toners are printed in a white part, so-called background smear occurs. When toner is cracked, the low melting point wax component inside the toner is exposed and when the toner having exposed wax passes a regulation blade, the toner adheres to the regulation blade, leading to accumulation of toner and external additive on the regulation blade, namely, so-called toner fixation occurs. When the toner fixes on the regulation blade, streaks where toners are not transferred on a developing roller occur, and white lines are generated in a part to be printed in a printed image.
As a method for increasing the rigidity of the resin, it is known that a small amount of polyfunctional monomers such as divinylbenzene and diacrylate compound is added so that the resin has a crosslinked structure and the crosslinked structures are uniformly formed in the toner. When the toner has many crosslinked points, the toner do not easily melt and not fix well. When the toner has a little crosslinked points, crosslinking is not effective. Thus, it is extremely difficult to design a toner having an appropriate crosslinked structure.
In terms of toner shape, a toner having an uneven shape easily causes a problem of the contamination of a photoconductor during process caused by silica added as a fluidizing agent having a weak adhesion force in a depression part, and silica migration to the depression part. In a dissolution-suspension method, a polyester resin capable of fixing at low temperature can be used, but a polymeric component may be necessarily added, and then liquid viscosity is increased and problems occur in productivity. An irregular uneven shape lacks charge stability, and moreover problems remain in durability and releasability.
To control electrostatic property of a toner, a charge control agent is added, but filming and a problem in fixing property occur. It has been proposed that when a toner is produced, a swellable clay substance is mixed and added so as to obtain a non-spherical toner, thereby improving cleaning property. A layered inorganic mineral is generally attracted as a charge control agent for a toner.
The layered inorganic mineral is an inorganic mineral having a microscopic structure in which layers of inorganic molecules are laminated and aggregated. A layer of inorganic molecule has a thickness of several Å to several nm, several hundred nm to several μm, and a configuration of one or a several laminated layers of a tetrahedral sheet in which tetrahedron having Si, Al and Fe in the center two dimensionally spreads, or a octahedral sheet in which octahedron having Al and Mg in the center two dimensionally spreads.
Examples of the layered inorganic minerals having only an octahedral sheet include brucite and hydrotalcite. Examples of the layered inorganic minerals having a 1:1 structure in which a tetrahedral sheet and an octahedral sheet are layered, include kaolin, serpentine and halloysite. Examples of the layered inorganic minerals having a 2:1 structure in which two tetrahedral sheets form on either side of an octahedral sheet include, talc, pyrophyllite, saponite, hectorite, montmorillonite, beidellite, vermiculite, natural mica, synthetic mica, lepidolite, illite, paragonite, clintonite, margarite, clinochlore, chamosite, nimite, donbassite, cookeite and sudoite.
An inorganic molecule layer may have positive or negative charge, depending on valence of metal atom constituting each layer. Anions and cations are intercalated in the interlayers and charges are canceled in total. In natural products, carbonate ions, inorganic ions such as Cl−, Na+, K+, Mg2+, Ca2+ are intercalated and have high hydrophilicity. Examples of the layers having positive charge (anionic layered inorganic minerals) include hydrotalcite, and examples of the layers having negative charge (cationic layered inorganic minerals) include saponite, hectorite, montmorillonite, beidellite, vermiculite, natural mica, synthetic mica, lepidolite, illite, paragonite, clintonite, margarite, clinochlore, chamosite, nimite, donbassite, cookeite and sudoite. As a result, between the laminated inorganic molecule layers, force having a charge such as an electrostatic force works in addition to van der Waals force. Hydrotalcite, hectorite, montmorillonite, beidellite, vermiculite and swellable mica have relatively weak electrostatic force, and some of them may be swelled by taking water molecules into interlayer, and finally the laminated structure thereof may be separated.
Moreover, in the layered inorganic mineral, an intercalated inorganic ion can be relatively easily exchanged with other ions, and can be exchanged with a lipophilic carboxylic acid and sulfonic acid such as alkyl carboxylic acid; ammonium cation and imidazolium cation. The inorganic mineral, in which an interlayer ion is replaced with a lipophilic ion is called as an organically modified inorganic mineral (Japanese Patent Application Laid-Open (JP-A) Nos. 2003-202708, 2006-500605 and 2006-503313).
When the layered inorganic mineral is added to a toner, a non-organically modified layered inorganic mineral has poor compatibility with a resin, and is detached from the toner. However, the layered inorganic mineral is organically modified to have higher oil solubility to be dispersed into the toner, thereby improving charge control property and heat stability.